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Old 25th September 2007, 11:21 PM   #1
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Default Open-Source Audio Input Switch

Hey everyone... I'm new around here, but I've been working on a small project to fit into a headphone amp I built, and as it's finally in full swing I figured I'd mention it here.

What I'm building is a small (up to) five-input audio input switch, where the audio is switched via relays. I suppose there's no good reason why this couldn't also be used to switch SPDIF.

I've just about wrapped up the software, and I expect the finished device it to have the following features:

- Full open-source hardware (some Creative Commons license, possibly non-commercial) and software (BSD license).
- Next / Previous buttons used to select input.
- LEDs to indicate selected output.
- Current input stored in microcontroller, so the selected input is preserved then restored when device is powered off and back on.
- Two or four pole relays can be fitted. Combined with three or five-place terminals this can then switch either balanced or unbalanced audio.
- All audio circuitry on PCB will be isolated from digital circuitry.
- Jumpers (solder-on, pin, or DIP switches) to select number of inputs currently in use.

I won't be selling completed devices, but I may offer pre-programmed microcontrollers and/or PCBs for those who don't have the facility to program PICs or etch boards themselves.

The software is functionally complete, I just have to polish it and bit and sort out the board design. Hopefully I'll have something more concrete in a week or so.

-Steve
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Old 27th September 2007, 07:56 PM   #2
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First draft of the software is done. It can be viewed here.

Please note that it's set up for my dev board, so the inputs will probably change. It's written in mikroBasic for a PIC16F630 running from its internal osc at 4MHz. IO config will likely change once I begin routing the board.

A full-on project page for this, the nuxx Audio Input Switch will be filled out as development progresses. The software itself is under the MIT license and and the board designs will be some manner of Creative Commons, possibly non-commercial. Once they are done, of course.

-Steve
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Old 3rd October 2007, 04:10 PM   #3
kevinkr is offline kevinkr  United States
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Sounds interesting, keep us posted.
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Old 3rd October 2007, 04:48 PM   #4
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by kevinkr
Sounds interesting, keep us posted.

Yup, likewise...
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Old 3rd October 2007, 05:48 PM   #5
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Okay, I think I've got the board layout set. Here is a teaser image of it.

Power:
Can be powered from DC (~7.5 - 35V), from AC (~5V - 27V), or from 5V DC. If powered via DC or AC input jacks so that the power is regulated down to 5VDC, 5VDC is available on the 5V connector for powering other devices.

Switch Connectors:
NX: Next Input
PR: Previous Input
MU: Mute Toggle

LED Connectors:
M: Indicates when mute is active.
PW: Power, always on.
I1 - I5: Indicate currently selected input.

The board can be used to switch either balanced or unbalanced audio. To switch balanced 4-pole relays should be fitted to as many inputs as desired, along with 5-place Phoenix MPT (or compatible) screw terminals. To switch unbalanced 2-pole relays and 3-place screw terminals.

The number of inputs used are selected by fitting jumpers (either normal jumpers, or small wire bridges can be used).

When pressing a button there is a 100ms delay between breaking all contact and enabling the next-selected input.

The system will save the currently selected input when the new input is selected, so when power is cut and restored the previously selected input is maintained.

The resistor network is used for current limiting of the LEDs. I suspect this will around 150Ω but is actually dependent on the type of front-panel indicator LEDs used.

Uhm... that's about it for now. I welcome any comments / suggestions that people have.
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Old 3rd October 2007, 06:10 PM   #6
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Oh, I should also mention that the board is originally designed to fit in a Hammond 1455T2202 along with a Millett Hybrid Maxed PCB, but will work just as well in another enclosure or in any of the other Hammond enclosures which will hold a 150mm wide PCB.

The PCB itself is 150mm x 50mm, and due to the non-profit license of EAGLE used for the layout, the PCB will be licensed Creative Commons Non-Commercial.

The software itself is MIT license. So, if you wanted to reimplement this software / board design in your own design, you may. You just can't make up boards and sell them for a profit. Of course, group buys of the PCBs are encouraged. I may also run one myself if there is sufficient interest...

Initial looking around shows the PCBs to be around US$12/each in small quantities and the microcontrollers are about US$1.30/each when purchased 25 at a time.

2-pole relays are US$3-something, but 4-pole are near US$9-something.

Today I'm going to order a set of sample parts for breadboarding / size checking. Once those come in, I've validated that the PCB layout will work, and I've finished making any changes which may crop up I'll grab an initial run of boards.
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Old 3rd October 2007, 09:02 PM   #7
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Okay, BOM is complete. For all parts (except the PCB, of course) from Mouser, set up to switch five balanced inputs the cost is $69.38. To switch five unbalanced inputs the cost is $37.73.

The PCBs are likely around $12/each. Of course a programmed microcontroller will be needed, but I can probably help out with that, or any cheap PIC programmer should do it.

The relays are almost half the cost of each implementation, so having fewer inputs could cut the cost down significantly. (5x 4PDT are US$43.85, 5x DPDT are US$17.50.)

I'm debating one last change, and that is to use discreet resistors instead of a resistor network, that way those who chose to use different color LEDs for the different indicators can use appropriate resistor values for each LED. In fact, I think I'll go do this now.
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Old 4th October 2007, 02:53 AM   #8
kevinkr is offline kevinkr  United States
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Took a look at the teaser layout - looks good..

I'd definitely be interested in a group buy if you get that far.
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Old 4th October 2007, 03:08 AM   #9
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Quote:
Originally posted by kevinkr
Took a look at the teaser layout - looks good..

I'd definitely be interested in a group buy if you get that far.
It's ready to go. I'd gladly handle the group buy for PCBs and programming the PICs, as long as there is sufficient interest.

I'd just have to etch a board for adapting the DIP12 to the 1x5 header of the programmer to make programming the chips easy on me. That'll only take a day or so, though.

I already have the software working great, so I just have to finalize the PCB (it's nearly done), ensure the power supply and LED/relay driver works as expected, and finalize resistor values in conjunction with some different LED selections.

Other than that it's really rather set. Oh, and pick a PCB fab house... I've got a few favorites, but the one I like to use for professional work isn't good for low quantity runs... Quality is great and they are in the US, but their prototype (~10 pieces) costs would be $30 - $50/board. I'll probably use pcbex.com or another Chinese fab front end again.

-Steve
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Old 4th October 2007, 09:11 PM   #10
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There. Parts for the prototype / breadboard / proof of concept have been ordered. I hope to have the parts early next week, and once I can validate that the driver and power supply work as expected I'll order the first run of five PCBs.
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